Conventional gasoline engines of automobiles not only emit pollutant emissions via combustion of fuel or via emission of lubricant or fuel in the crankcase, the engines also produces hydrocarbon emissions via evaporation of fuel stored in the automobile. To reduce or eliminate this form of emission, modern automobiles store the fuel vapor in a canister and control its release from the canister into the combustion chamber for combustion. With reference to FIG. 1 such on-board evaporative emission control system (EVAP), generally indicated at 10, typically includes a fuel vapor collection canister (e.g., a carbon canister) 12 and a normally closed canister purge valve 14 connected between a fuel tank 16 and an intake manifold 18 of an internal combustion engine 20 in a known fashion. A normally open canister vent valve 22 is in fluid communication between a vapor collection canister 12 and ambient atmospheric conditions via a filter 24. Under certain conditions, the purge valve 14 is opened to direct hydrocarbon vapors to the intake manifold 18 to be consumed by the engine 20. The system of FIG. 1 works well, but better control of the air/fuel ratio is needed.
Thus, there is a need in an evaporative emission control system to provide sensor that monitors both hydrocarbon concentration and flow rate of gas flow in the system to improve an air/fuel ratio of an engine.